Hydrolytically degradable carbamate derivatives of poly (Ethylene Glycol)

ABSTRACT

Poly(ethylene glycol) carbamate derivatives useful as water-soluble pro-drugs are disclosed. These degradable poly(ethylene glycol) carbamate derivatives also have potential applications in controlled hydrolytic degradation of hydrogels. In such degradable hydrogels, drugs may be either trapped in the gel and released by diffusion as the gel degrades, or they may be covalently bound through hydrolyzable carbamate linkages. Hydrolysis of these carbamate linkages releases the amine drug at a controllable rate as the gel degrades.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/033,547, filed Jan. 10, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/361,451 filed on Feb. 10, 2003, now U.S. Pat.No. 6,899,867, which is a continuation of U.S. application Ser. No.09/842,514 filed on Apr. 26, 2001, now U.S. Pat. No. 6,541,015, which isa divisional of U.S. application Ser. No. 09/469,418 filed on Dec. 23,1999, now U.S. Pat. No. 6,413,507, all of which are herein incorporatedby reference in their entireties.

FIELD OF THE INVENTION

This invention relates to hydrolyzable derivatives of poly(ethyleneglycol) useful as prodrugs and as degradable components of cross-linkedpolymers.

BACKGROUND OF THE INVENTION

Covalent attachment of the hydrophilic polymer, poly(ethylene glycol),commonly referred as PEG, to biologically active agents and surfaces hasimportant applications in biotechnology and medicine.

PEG is generally soluble in water and many organic solvents. PEG is alsosubstantially non-toxic and normally does not illicit any significantimmune response in animals. When PEG is chemically attached to a waterinsoluble compound, the resulting conjugate generally is soluble inwater as well as many organic solvents. When the agent to which PEG isattached is biologically active, such as a drug, the activity of theagent can be retained after the attachment of PEG, and the conjugategenerally displays altered pharmacokinetics.

The prodrug approach, in which drugs are released by degradation of morecomplex agents (prodrugs) under physiological conditions, is a powerfulcomponent of drug delivery. See R. B. Greenwald, Exp. Opin. Ther.Patents, 7(6):601-609 (1997). Prodrugs can, for example, be formed bybonding PEG to drugs using linkages which are degradable underphysiological conditions.

However, not all linkages are readily degradable and useful in prodrugapplicaitons. In general, ester linkages, formed by condensationreactions between PEG carboxylic acids or activated PEG carboxylic acidsand alcohol groups on drugs, hydrolyze under physiological conditions torelease the drug. For example, in PCT Publication No. WO 96/23794, it isdisclosed that paclitaxel can be linked to PEG using ester linkages andthe linked paclitaxel can be released in serum by hydrolysis.Antimalarial activity of dihydroartemisinin bonded to PEG through ahydrolyzable ester linkage has also been demonstrated. Bentley et al.,Polymer Preprints, 38(1):584 (1997).

Conventional amide and carbamate linkages, formed with amine groups ondrugs, generally are stable and do not hydrolyze to release a free drugwithin a sufficiently short time that is required in practicalapplications. See, e.g., Zalipsky, Advanced Drug Delivery Reviews,16:157-182 (1995); Zalipsky, et al., Eur. Polym. J., 19:1177-1183(1983). For example, it has been demonstrated that carbamate linkagesbetween PEG and a protein in a conjugate are stable under a variety ofphysiological conditions. Larwood and Szoka, J. Labeled Compd.Radiopharm. 21:603 (1984). Many useful drugs including peptides,proteins, and small agents having amine groups have been bonded to PEGthrough non-hydrolyzable amide and carbamate linkages. PEG can also bebonded to amine groups on drugs through reductive amination with PEGaldehydes and the resulting amine linkage is non-degradable in vivo.

Because many drugs such as proteins have amine groups that are readilyavailable for reaction to form linkages, it is desirable to make suchlinkages hydrolytically degradable so that free proteins or otheramine-containing agents can be released from the prodrugs at acontrolled rate in vivo. Imines, or Schiff bases, offer a possibleapproach since they hydrolyze to generate the free amine and analdehyde:

where R′ is a drug or other agent bearing an amino group. This approachhas been used in attaching doxorubicin to PEG with release of the drugoccurring by hydrolysis of the imine linkage. Ouchi et al. PolymerPreprints, 38(1):582-3 (1997). Since the formation of imines isreversible in water, these compounds are best prepared in organicsolvents. Many proteins, peptides, and other agents are thus notamenable to the imine prodrug approach because of their poor solubilityor instability in organic solvents.

Conjugates can be prepared by linking an amine-containing drug, througha non-hydrolyzable amide or carbamate linkage, to a PEG molecule havinghydrolytically degradable linkages in the PEG backbone. Theamine-containing drug is releasable upon the degradation of the PEGbackbone. However, the released drug usually has a fragment attachedthrough an amide or carbamate linkage, and the native or parent drug isnot released.

U.S. Pat. No. 4,935,465 discloses a water-soluble prodrug in whichneighboring group participation by a carboxyl group aids in thehydrolysis of an amide, thus releasing the drug. PEG was a component ofa bovine serum albumin (BSA) prodrug disclosed in that patent:

U.S. Pat. No. 5,561,119 and European Patent No. 595133-A disclose adoxorubicin prodrug as shown below, which utilizes a benzylglucuronylcarbamate linkage. A second component, glucuronidase, must be added inorder to cleave the glucuronic acid and liberate doxorubicin and anitrobenzoquinone methide.

In yet another approach as disclosed in U.S. Pat. No. 5,413,992, aprodrug of daunamycin shown below, liberates the native drug by anenzyme-induced elimination initiated by abstraction of a proton adjacentto the sulfone group.

In addition, U.S. Pat. No. 4,760,057 describes enzymatic hydrolysis of aprodrug containing a carbamate linkage:RR′NCO₂CR₁R₂O₂CR₃where RR′N represents the secondary amine on a drug moiety, and R₁₋₃ arevarious moieties such as hydrogen, alkyls, or cycloalkyls. Such prodrugsare hydrolyzed by esterases to generate RR′NCO₂CR₁R₂OH which thendecomposes to liberate the drug agent.

Greenwald et al. J. Med. Chem., 42:3657-3667 (1997) discloses prodrugshaving a drug linked, through a carbamate linkage to a PEG derivative.1,4 or 1,6 elimination reaction is required to release the free drug.The prodrug is structurally complex and toxic quinone methideintermediates may be liberated along the free drug.

Thus, the prodrugs in the prior art generally have drawbacks that limittheir practical applications. The requirement for enzyme digestion makesthe prodrugs unsuitable or at least less useful for in vivo use. Inaddition, the generation of toxic intermediates can be associated withthe release of free drugs. Thus, there remains a need for prodrugshaving improved characteristics.

SUMMARY OF THE INVENTION

The invention provides a water soluble prodrug in which a biologicallyactive agent is linked to a water soluble non-immunogenic polymer by ahydrolyzable carbamate bond. The biologically active agent can bereadily released by the hydrolysis of the carbamate bond in vivo withoutthe need for adding enzymes or catalytic materials. Generally, thebiologically active agent is released, upon hydrolysis, into its parentstate, i.e., without any additional moieties attached thereto. Inaddition, because a water soluble, non-peptidic polymer is used, even asubstantially insoluble biologically active agent can be readilydelivered in the prodrug in vivo.

Thus, in accordance with the present invention, a prodrug is providedhaving the formula:

wherein POLY is a water soluble and non-peptidic polymer, L is a linkinggroup, Ar is an aromatic group, and Y is a biologically active agent.

The water soluble non-immunogenic polymer can have a capping groupselected from the group consisting of OH, alkoxy, and

wherein L′ is a linking group, Ar′ is an aromatic group, and Y′ is abiologically active agent. Preferably, POLY is a poly(ethylene glycol)or a derivative thereof having a molecular weight of from about 200 toabout 100,000 Daltons.

In accordance with another embodiment of the invention, a compound isprovided having the formula:

in which POLY is a water soluble, non-peptidic polymer, L is a linkinggroup, Ar is an aromatic group, and X is an activating group capable ofreacting with an amino group of a biologically active agent to form acarbamate linkage.

Optionally, POLY can have a capping group selected from the groupconsisting of OH, alkoxy, and

wherein L′ is a linking group, Ar′ is an aromatic group, and X′ is anactivating group capable of reacting with an amino group of abiologically active agent to form a carbamate linkage. Preferably, POLYis a poly(ethylene glycol) or a derivative thereof having a molecularweight of from about 200 to about 100,000 Dalton.

In another embodiment of this invention, a prodrug is provided havingthe formula:Y—Ar—O₂C—NH—POLYwhere Y is a biologically active agent having an aromatic group, Ar isthe aromatic group of the biologically active agent Y, such as asubstituted benzene or other aromatic such as a substituted naphthaleneor heterocylic moiety, and POLY is a water soluble, non-petidic polymer,preferably poly(ethylene glycol) in any of its form. Hydrolysis of thisderivative yields the parent drug Y—ArOH, and POLY—NH₂ and CO₂.

In accordance with yet another embodiment of the present invention, ahydrolytically degradable hydrogel is provided. The hydrogel comprises abackbone bonded to a crosslinking agent through a hydrolyzable carbamatelinkage. Typically, a suitable backbone can be any compound having anamino group, preferably at least two amino groups. Examples of suchbackbones include, but are not limited to, proteins, peptides,aminocarbohydrates, aminolipids, poly(vinylamine), polylysine,poly(ethylene glycol) amines, pharmaceutical agents having an aminogroup, etc. The crosslinking agent is selected from the group consistingof:

wherein POLY is a non-peptidic, water soluble polymer, L and L′ arelinking groups, Ar and Ar′ are aromatic groups, Z is a central branchcore, n is from 2 to about 100, and X and X′ are activating groupscapable of reacting with the amino groups in the backbone to formhydrolyzable carbamate linkages. Preferably, POLY is a poly(ethyleneglycol) or derivative thereof having a molecular weight of from about200 to about 100,000.

The foregoing and other features and advantages of the invention, andthe manner in which the same are accomplished, will be more readilyapparent upon consideration of the following detailed description of theinvention in conjunction with the claims and the drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a CE graph showing the hydrolysis of mPEG-lysozyme conjugateprepared with N-mPEG benzamide-m-succimidyl carbonate. At time zero, asmall amount of free lysozyme was mixed with mono, di, and tri PEGylatedlysozyme (Curve A). After hydrolysis for 10 days at pH 7 and 37° C.,more than 85% of free lysozyme was released (Curve B). Peaks I, II, III,and IV represent free lysozyme, mono-PEGylated lysozyme, di-PEGylatedlysozyme and tri-PEGylated lysozyme, respectively.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “prodrug” means a chemical derivative of abiologically active agent which can release or liberate the parentbiologically active agent under defined conditions. By converting aparent biologically active agent into a prodrug, the solubility andimmunogenicity of the agent can be modified. In addition, by controllingthe rate of release of the agent from the prodrug, temporal control ofthe agent's action in vivo can be achieved.

The term “biologically active agent” when used herein means anysubstances which can affect any physical or biochemical properties of abiological organism including but not limited to viruses, bacteria,fungi, plants, animals and humans. In particular, as used herein,biologically active agent includes any substance intended for thediagnosis, cure, mitigation, treatment, or prevention of disease inhumans or other animals, or to otherwise enhance physical or mental wellbeing of humans or animals. Examples of biologically active agentsinclude, but are not limited to, organic and inorganic compounds,proteins, peptides, lipids, polysaccharides, nucleotides, DNAs, RNAs,other polymers, and derivatives thereof. Examples of biologically activeagents also include, e.g., antibiotics, fungicides, anti-viral agents,anti-inflammatory agents, anti-tumor agents, cardiovascular agents,anti-anxiety agents, hormones, growth factors, steroidal agents, and thelike.

A prodrug of this invention has the formula:

wherein:

-   -   POLY is a substantially non-immunogenic water soluble polymer;    -   L is a covalent linkage, preferably a hydrolytically stable        linkage;    -   Ar is an aromatic group; and    -   Y is a biologically active agent.

As used herein, the terms “group,” “functional group,” “active moiety,”“reactive site,” reactive groups” and “reactive moiety” are all somewhatsynonymous in the chemical arts and are used in the art and herein torefer to distinct, definable portions or units of a agent and to unitsthat perform some function or activity and are reactive with otheragents or portions of agents.

The term “linking group” is used to refer to groups that normally areformed as the result of a chemical reaction and typically involvecovalent bonding.

In the prodrug of this invention, the substantially water solublenon-immunogenic polymer POLY is preferably poly(ethylene glycol) (PEG).However, it should be understood that other related polymers are alsosuitable for use in the practice of this invention and that the use ofthe term PEG or poly(ethylene glycol) is intended to be inclusive andnot exclusive in this respect.

Poly(ethylene glycol) or PEG is useful in biological applicationsbecause it has properties that are highly desirable and is generallyapproved for biological or biotechnical applications. PEG typically iscolorless, odorless, soluble in water, stable to heat, inert to manychemical agents, does not hydrolyze or deteriorate, and is generallynontoxic. Poly(ethylene glycol) is considered to be biocompatible, whichis to say that PEG is capable of coexistence with living tissues ororganisms without causing harm. More specifically, PEG normally does nottend to produce an immune response in the body. When attached to anagent having some desirable function in the body, the PEG tends to maskthe agent and can reduce any immune response so that an organism cantolerate the presence of the agent. Accordingly, the prodrug of theinvention typically is substantially non-toxic and does not tend toproduce substantial immune response or cause clotting or otherundesirable effects. PEG having the formula —(CH₂CH₂O)_(n)—CH₂CH₂—,where n is from about 8 to about 4000, is one useful polymer in thepractice of the invention. Preferably PEG having a molecular weight offrom about 200 to about 100,000 Da is used as POLY.

In its most common form, PEG is a linear polymer having a hydroxyl groupat each terminus:HO—CH₂—CH₂O(CH₂CH₂O)_(n)CH₂CH₂—OH

PEG is commonly used as methoxy-PEG, or mPEG in brief, in which oneterminus is the relatively inert methoxy group, while the other terminusis a hydroxyl group that is subject to ready chemical modification:CH₃O—(CH₂CH₂O)_(n)—CH₂CH₂—OH

Branched PEGs are also in common use. The branched PEGs can berepresented as R(—PEG—OH)_(m) in which R represents a central core agentsuch as pentaerythritol or glycerol, and m represents the number ofarms. The number of arms m can range from three to a hundred or more.The hydroxyl groups are subject to ready chemical modification.

Another branched form of PEG can be represented as (CH₃O—PEG—)_(p)R—Z,where p equals 2 or 3, R represents a central core such as lysine orglycerol, and Z represents a group such as carboxyl that is subject toready chemical activation. This type of PEG has a single terminus thatis subject to ready chemical modification.

Yet another branched form, the pendant PEG, has reactive groups, such ascarboxyls, along the PEG backbone rather than at the end of PEG chains.

Forked PEG represented by the formula PEG(—LCHX₂)_(n) is another form ofbranched PEG, where L is a linking group and X is an activated terminalgroup.

In addition, the polymers can also be prepared to have weak ordegradable linkages in the backbone. For example, PEG havinghydrolytically unstable ester linkages in the polymer backbone can beprepared. The ester linkages are susceptible to hydrolysis which resultsin cleavage of the polymer into fragments of lower molecular weight:

It is understood by those skilled in the art that the term poly(ethyleneglycol) or PEG represents or includes all the above forms.

Other polymers than PEG are also suitable for the present invention.These other polymers include, but are not limited to, otherpoly(alkylene oxides) such as poly(propylene glycol) (“PPG”), copolymersof ethylene glycol and propylene glycol and the like; poly(oxyethylatedpolyols) such as poly(oxyethylated glycerol), poly(oxyethylatedsorbitol), and poly(oxyethylated glucose); poly(vinyl alcohol) (“PVA”);dextran; carbohydrate-based polymers and the like. The polymers can behomopolymers or random or block copolymers and terpolymers based on themonomers of the above polymers, straight chain or branched.

Specific examples of suitable additional polymers include, but are notlimited to, poly(oxazoline), difunctional poly(acryloylmorpholine)(“PAcM”), and poly(vinylpyrrolidone)(“PVP”). PVP and poly(oxazoline) arewell known polymers in the art and their preparation should be readilyapparent to the skilled artisan. PAcM and its synthesis and use aredescribed in U.S. Pat. Nos. 5,629,384 and 5,631,322, the contents ofwhich are incorporated herein by reference in their entirety.

Although the molecular weight of POLY can vary, it is typically in therange of from about 100 to about 100,000, preferably from about 2,000 toabout 80,000.

Those of ordinary skill in the art will recognize that the foregoinglist for substantially water soluble non-immunogenic polymer POLY is byno means exhaustive and is merely illustrative, and that all polymericmaterials having the qualities described above are contemplated.

The polymer POLY can have a terminal capping group distal to thebiologically active agent Y. Examples of the capping group include, butare not limited to, OH, alkoxy, and

wherein L′ is a hydrolytically stable linkage, Ar′ is an aromatic group,and Y′ is a biologically active agent. L′, Ar′, and Y′ can be same ordifferent from L, Ar, and Y respectively.

The aromatic groups Ar and Ar′ in the prodrug can be any aryl groups inany chemically arranged forms. For example, phenyl, substituted phenyl,biphenyl, substituted biphenyl, polycyclic aryls, substituted polycyclicaryls, heterocyclic aryls, substituted heterocylic aryls, andderivatives thereof can all be used. The substitutions on the aromaticring(s) of Ar and Ar′ can be at any position relative to L or L′.Examples of suitable substitution moieties include, but are not limitedto, halogen, alkyls, alkoxy, hydroxy, carboalkoxy and carboxamide. Itshould be understood that these additional groups bonded to the aromaticgroup may affect the hydrolysis rate of the carbamate linkage between Arand Y, and/or Ar′ and Y′. Thus, different substitution moieties can bechosen to control the release rate of the biologically active agent Yand Y′. Preferably Ar and Ar′ are benzenes or substituted benzenes.

The linking groups L and L′ link the aromatic groups Ar and Ar′,respectively, to the non-immunogenic polymer POLY. Typically they areformed by reacting a terminal group of POLY with a reactive moiety on aring of the aromatic group Ar or Ar′. L and L′ can be any covalentlinkages. In particular, L and L′ can include covalent bonds such asethers, amines, imines, imides, amides, carbamides, esters, thioesters,carbonates and ureas. For example, L and L′ can be selected frommoieties such as —O—, —NR— where R is H, a C₁₋₆ alkyl or substitutedalkyl, —CO₂—, —O₂C—, —O₂CO—, —CONH—, —NHCO—, —S—, —SO—, —SO₂—, etc.Preferably L and L′ are —O—, or —NHCO—.

The carbamate linkages between Ar and Y, and Ar′ and Y′ are hydrolyzablein vivo at a desirable rate. Typically, when a prodrug of this inventionis delivered into the body, the prodrug is first delivered to thedesired tissue or organ through a selected route, e.g., bloodcirculation. The parent biologically active agent is released byhydrolysis. Once the parent agent is released, the rest of thecomponents of the prodrug are subsequently eliminated by biodegradationor excretion. To achieve the optimal result, the linkages L and L′typically are more stable than the hydrolyzable carbamate linkage.Preferably, L and L′ are hydrolytically stable linkages. In addition,the prodrug circulation lifetime should be longer than the time requiredfor hydrolysis of the carbamate linkage.

In the prodrug of this invention, the release rate of the parentbiologically active agent from the prodrug can be modified in a numberways. It has been found that the rate of hydrolytic degradation of thecarbamate linkage is affected by the position of the attachment of the Lor L′, as defined above, to the aromatic ring relative to the positionof the carbamate linkage attachment. That is, the carbamate hydrolysisrates vary, in the case of benzene derivatives, between ortho, meta, andpara placement of L or L′. The rate of hydrolysis of the carbamatelinkage is also affected by the nature of L and L′, for example an etherlinkage is more stable than an amide linkage. Moreover, additionalmoieties bonded to the aromatic group may affect the hydrolysis rate ofthe carbamate linkage. Thus, different substitution moieties can bechosen to control the release rate of the biologically active agent Yand Y′.

In one preferred embodiment, the prodrug of this invention has theformula:

wherein:

-   -   L is —O— or —NHCO—;    -   Y is a biologically active agent;

POLY is poly(ethylene glycol) having a capping group selected from thegroup consisting of —OH, C₁₋₄ alkyl, and

wherein Y′ and L′ are as described above.

Thus, the hydrolysis of the carbamate linkage in the prodrug can beillustrated as follows:

Although, the present invention is especially suited for deliveringbiologically active agents that are water insoluble and/or immunogenic,this invention can be used for virtually any biologically active agents.However, as is clear below in the description of the synthesis of theprodrug, the biologically active agent to be converted to the prodrug ofthis invention must have an amino group or a moiety that can beconverted to an amino group. Suitable biologically active agentsinclude, but are not limited to, proteins, enzymes, peptides,aminolipids, polysaccharides having an amino group,amino-oligonucleotides, and pharmaceutical agents having an amino group.

Generally the method of synthesizing a prodrug of this inventionincludes the following steps: first, an activated water soluble andnon-peptidic polymer is provided. The activated polymer typically has areactive terminal moiety. For example, the activated polymer can bePOLY—NH₂, H₂N—POLY—NH₂, POLY—O—SO₂—CH₃, or CH₃—SO₂—O—POLY—O—SO₂—CH₃, andthe like. An aryl compound having two reactive substitution groupslinked to the aromatic ring is also provided. The aryl compound can be,e.g., hydroxybenzoic acid or benzyloxyphenol. One of the two reactivegroups on the aromatic ring can react with the reactive terminal moietyof the activated polymer to form the linkage L. The other reactive groupof the aryl compound either itself can react with an amino group of abiological active agent to form a hydrolyzable carbamate linkage, or canbe converted into a reactive group which can react with an amino groupof a biological active agent to form a hydrolyzable carbamate linkage.Thus, a compound is provided having the formula:

wherein POLY, L, and Ar are as described in regard to the prodrug ofthis invention, and wherein X is an activating group capable of reactingwith an amino group of a biologically active agent to form ahydrolyzable carbamate linkage.

Preferably, L is —O— or —NHCO—, Ar is a substituted or unsubstitutedbenzene moiety, X is chlorine, bromine, N-succinimidyloxy, or1-benzotriazolyloxy, and POLY is poly(ethylene glycol) or a derivativethereof with a molecular weight of from about 200 to about 100,000Dalton and having a capping group selected from the group consisting of—OH, C₁₋₄ alkyl, and

where L′ is —O— or —NHCO—, Ar′ is a substituted or unsubstituted benzenemoiety, and X′ is chlorine, bromine, N-succinimidyloxy, or1-benzotriazolyloxy.

In another embodiment of this invention, a prodrug is provided havingthe formula:Y—Ar—O₂C—NH—POLYwhere Y is a biologically active agent having an aromatic group, Ar isthe aromatic group of the biologically active agent Y, such as asubstituted benzene or other aromatic such as a substituted naphthaleneor heterocylic moiety, and POLY is a water soluble, non-petidic polymeras described above, preferably poly(ethylene glycol) in any of its form.Hydrolysis of this derivative yields the parent drug Y—ArOH, andPOLY—NH₂ and CO₂.

In accordance with another aspect of this invention, a hydrolyticallydegradable hydrogel is provided. The hydrogel comprises a backbonebonded to a crosslinking agent through a hydrolyzable carbamate linkage.

Typically, the backbone of the hydrogel is a biocompatiblemacromolecule. The backbone has an amino group available to react withthe crosslinking agent to form a hydrolyzable carbamate linkage.Preferably, the backbone has at least two of such amino groups. Examplesof such backbones include, but are not limited to, proteins, modifiedproteins such as glycoproteins, phosphorylated proteins, acylatedproteins, and chemically modified proteins, peptides,aminocarbohydrates, glycosaminoglycans, aminolipids, poly(vinylamine),polylysine, poly(ethylene glycol) amines, pharmaceutical agents havingat least two amino groups, etc. Specific examples of the backboneinclude, but are not limited to, fibrin, fibrinogen, thrombin, albumins,globulins, collagen, fibronectin, chitosan and the like. In addition,the backbone may also be microorganisms such as viral particles,bacterial cells, or animal or human cells.

The crosslinking agent can be the difunctional polymer described abovehaving the formula:

wherein POLY, POLY′, L, L′, X, X′, Ar, and Ar′ are as described above.Alternatively, the crosslinking agent can also be a branchedwater-soluble substantially non-immunogenic polymer having the formula:

wherein POLY, L, L′, Ar, Ar′, X and X′ are as described above. Z is acentral branch core moiety. n represents the number of arms and is from2 to about 100. In particular, the central branch core moiety can bederived from the amino acid lysine, or polyols such as glycerol,pentaerythritol and sorbitol. Branched PEGs are known in the art.Suitable branched PEGs can be prepared in accordance with U.S. Pat. No.5,932,462, which is incorporated herein in their entirety by reference.These branched PEGs can then be modified in accordance with the presentteachings. For example, a four-arm, branched PEG prepared frompentaerythritol is shown below:C(CH₂—OH)₄+n C₂H₄O —6 C[CH₂O—(CH₂CH₂O)_(n)—CH₂CH₂—OH]₄

This branched PEG can then be further modified to form the branchedcrosslinking agent by the method as described above in the context ofsynthesizing a prodrug.

In a preferred embodiment, the crosslinking agent has the formula:

wherein X and L are as described above. Thus, the crosslinking of abackbone having multiple amino groups by this crosslinking agent in theprocess for forming a hydrogel can be illustrated as follows:

where the zig-zag notation represents a backbone having amine groups andwhere L is as described above.

As will be apparent, the carbamate linkages between the backbones andthe crosslinking agents formed from the crosslinking reactions arehydrolyzable. Thus, the hydrogel of this invention can gradually breakdown or degrade in the body as a result of the hydrolysis of thecarbamate linkages. Therefore, the hydrogel of this invention can beused as a carrier for delivery of biologically active agents and othersuitable biomedical applications. For example, the hydrogel can carrytherapeutic drugs and can be implanted or injected in the target area ofthe body. The hydrogel may also carry other agents such as nutrients orlabeling agents for imaging analysis.

In the various applications of the hydrogel of this invention, thebiologically active agents to be delivered can be used as the backbone,or part of the backbone of the hydrogel. Alternatively, biologicallyactive agents can be in the form of a prodrug as described above andcovalently linked to the hydrogel as illustrated:

wherein L is a linkage as described above, Y is a biologically activeagent to be delivered in the hydrogel. Typically, in this case, Y has anamino group which can react and form a carbamate linkage as describedabove. Also, biologically active agents or other substances to bedelivered can also be loaded into the hydrogel during the synthesis ofthe hydrogel, or afterwards, e.g., by diffusion into the cavity ormatrix of the hydrogel without being covalently bonded to the hydrogelstructure, that is, the backbone or crosslinking agent of the hydrogel.

Because the crosslinking agents in the hydrogel are water soluble andsubstantially non-immunogenic, the hydrogel can be substantially watersoluble and non-immunogenic as well. In addition, because of theinterconnection by a large number of hydrolytically degradable carbamatelinkages, typically the degradation or breakdown of the hydrogel in thebody is gradual in nature. Thus, it is particularly useful for sustainedrelease of a biologically active agent or other substances in the body.

The present invention is further illustrated in the following exampleswhich are given to illustrate the invention, but should not beconsidered in limitation of the invention.

EXAMPLES Example 1 Synthesis of N-mPEG benzamide-m-succinimidylcarbonate (1)

mPEG amine 5000 (1.5 g, 0.3 mmole), 3-hydroxybenzoic acid (44 mg, 0.315mmole) and dicyclohexylcarbodiimide (DCC, 84 mg) were dissolved in 20 mlof anhydrous THF. The solution was stirred at room temperatureovernight. The solvent was condensed to half on a rotary evaporator andthe residue was precipitated into 150 ml of ethyl ether. The precipitatewas collected by filtration and dried in vacuo. Yield 1.5 g (100%). ¹HNMR(DMSO-d₆):δ3.5 (br m, PEG), 6.90 (m,aromatic), 7.22 (m, aromatic),8.37 (t, PEG—NHCO—), 9.62 (s, —C₆H₆—OH).

The above product (1 gram) and disuccinimidyl carbonate (DSC, 200 mg)were dissolved in 8 ml of acetonitrile. To the solution was added 200 ulof pyridine. The solution was stirred under nitrogen overnight and thesolvent was removed under reduced pressure. The resulting solid wasredissolved in 10 ml of dry chloroform and the insoluble solid wasremoved by filtration. The solution was then precipitated into 150 ml ofdry ethyl ether and the precipitate collected by filtration and dried invacuo. Yield 0.95g (95%). ¹H NMR(DMSO-d₆):δ3.5 (br m, PEG), 7.58 (m,aromatic), 7.83 (m, aromatic), 8.64 (t, PEG—NHCO—).

Example 2 Synthesis of N-mPEG-benzamide-p-succinimidyl carbonate (2)

mPEG amine 5000 (3 g, 0.6 mmole), 4-hydroxybenzoic acid (87 mg, 0.62mmole) and dicyclohexylcarbodiimide (DCC, 160 mg) were dissolved in 20ml anhydrous THF. The solution was stirred at room temperatureovernight. The solvent was condensed to half on a rotary evaporator andthe residue was precipitated into 150 ml of ethyl ether. The precipitatewas collected by filtration and dried in vacuo. Yield 3 g (100%). ¹HNMR(DMSO-d₆):δ3.5 (br m, PEG), 6.78 (d, aromatic), 7.70 (d, aromatic),8.23 (t, PEG—NHCO— ), 9.94 (s, —C₆H₆—OH).

The above product (1.5 gram) and disuccinimidyl carbonate (DSC, 300 mg)were dissolved in 12 ml of acetonitrile. To the solution was added 300ul of pyridine. The solution was stirred under nitrogen overnight andthe solvent was removed under reduced pressure. The resulting solid wasredissolved in 10 ml of dry chloroform and the insoluble solid wasremoved by filtration. The solution was then precipitated into 150 ml ofdry ethyl ether. The precipitate was collected by filtration and driedin vacuo. Yield 1.42 g (95%). ¹H NMR(DMSO-d₆):δ3.5 (br m, PEG), 7.49 (d,aromatic), 7.95 (d, aromatic), 8.60 (t, PEG—NHCO—).

Example 3 Synthesis of mPEG phenyl ether-p-succinimidyl carbonate (3)

mPEG mesylate 5000 (5 g, 1 mmole) in 60 ml of toluene was azeotropicallydistilled under nitrogen. After two hours, the solution was cooled toroom temperature.

4-benzyloxyphenol (0.44 g, 2.2 mmole) was added to a mixture of 0.46 mlof sodium methoxide (2 mmole, 25% in methanol) and 25 ml of drymethanol. The mixture was slowly stirred under nitrogen for 20 minutes.Methanol was then gradually distilled off until about 5 ml of solutionwas left. 50 ml of dry toluene was added and the solution was distilledunder nitrogen. The azeotropic distillation was not stopped until allmethanol was removed. The mixture was cooled to room temperature. Thefreshly azeotropically dried mPEG mesylate from the previous step wasadded and the mixture was refluxed under nitrogen overnight. Thereaction mixture was cooled to room temperature, toluene was distilledoff, and methylene chloride was added. The solid was removed byfiltration and the filtrate was washed with 10% sodium bicarbonatecontaining 10% sodium chloride aqueous solution and then dried oversodium sulfate. The dry methylene chloride solution was filtered,condensed on a rotary evaporator and precipitated into 100 ml of ether.The product was collected by filtration and dried in vacuum. Yield 4.5 g(90%). ¹H NMR(DMSO-d₆):δ3.5 (br m, PEG), 4.00 (t, —PEGOCH₂CH₂OC₆H₄O—),5.02 (s, —PEGOC₆H₄OCH₂C₆H₅), 6.90 (d+d, —PEGOC₆H₄O—), 7.35 (m,—PEGOC₆H₄OCH₂C₆H₅).

mPEG —p-(benzyloxy)-phenyl ether (4.5 g,0.9 mmole) was dissolved in1,4-dioxane (40 ml), and then hydrogenated with H₂ (2 atm pressure) and1.5 gram Pd/C (10%) overnight. The catalyst was removed by filtrationand the product precipitated into ethyl ether after most solvent wasdistilled off on a rotary evaporator. Yield: 3.7 gram (82%). ¹HNMR(DMSO-d₆):δ3.5 (br m, PEG), 3.96 (t, —PEGOCH₂CH₂OC₆H₄OH), 6.70 (d+d,—PEGOC₆H₄O—), 8.89 (s, —OH).

mPEG phenyl ether-p-phenyl alcohol (1.2 g) and disuccimidyl carbonate(DSC, 210 mg) were dissolved into 15 ml of acetonitrile. To the solutionwas added 0.12 ml of pyridine. The solution was stirred under nitrogenovernight and the solvent was removed under reduced pressure. Theresulting solid was redissolved in 10 ml of dry chloroform and theinsoluble solid was removed by filtration. The solution was thenprecipitated into 150 ml of dry ethyl ether. The precipitate wascollected by filtration and dried in vacuo. Yield 1.15 gram. (96%). ¹HNMR(DMSO-d₆):δ3.5 (br m, PEG), 7.49 (d, aromatic), 7.95 (d, aromatic),8.60 (t, PEG—NHCO—).

Example 4 Preparation of mPEG—NH—COO-Drug

20 mg of the above drug was azeotropically dried in pyridine andmethoxy-PEG isocyanate (177 mg, 5000 Dalton) was then added. Thesolution was stirred at room temperature overnight and the solvent wasremoved under reduced pressure to yield a residual syrup. To this wasadded 100 ml of ether and the resulting precipitate was collected byfiltration and dried in vacuo. PEG conjugation was demonstrated to be60% by ¹H NMR and GPC.

Example 5 Synthesis of mPEG phenyl ether-p-mexiletine carbamate

mPEG phenyl ether-p-succinimidyl carbonate (300 mg, 5000 Dalton), andmexiletine hydrochloride (16 mg), TEA (20 μl) were disclosed in 8 ml ofanhydrous methylene chloride. The solution was stirred overnight. Thesolvent was condensed on a rotary evaporator and 100 ml of isppropylalcohol was added to the residual syrup. The resulting precipitate wascollected by filtration, washed with 20 ml of ether, and dried in vacuo.¹H NMR(DMSO-d₆):δ3.5 (br m, PEG) 2.23 (s, CH3—), 6.9 (M, aromatic H),1.23 (d, —CH2—CH(CH3)—). Conjugation was shown to be greater than 90% byGPC.

Example 6 Modification of Lysozyme with the PEG Derivatives in Examples1-3

5-25 mg of each of the PEG derivatives prepared in Examples 1-3 wasmixed with 1 ml of lysozyme solution at pH 7 (5 mg/ml in 0.1 M phosphatebuffer). The solution was gently shaken for 5 hours at room temperature,and then stored at +4° C. for future analysis. PEGylation was monitoredby capillary electrophoresis.

Example 7 Monitoring Hydrolysis of the PEG Conjugate of Lysozyme byCapillary Electrophoresis

The conjugates prepared as described above were placed at 37° C. and atroom temperature and hydrolysis was monitored by capillaryelectrophoresis (CE). The CE graphs are shown in FIG. 1.

CE conditions: A solution of 25 mM phosphate buffer, containing 0.1mg/ml PEO 600K, pH 2.7 was flushed through the capillary forapproximately 15-20 min. A voltage of 15 kV was applied until a smoothbaseline was obtained. The 25 mM phosphate buffer solution was againflushed through for approximately 5 min and the capillary was then readyfor sample injection. The sample, which was adjusted to pH 2 by aphosphate buffer (0.1 M, pH 2), was injected hydrostatically for about10 sec at a height of approximately 6 inches. A voltage of 15 kV wasapplied throughout the run with a current between 24 and 30 μA. Theprotein and PEG-protein conjugate were detected by a UV monitor at 214nm. The CE instrument consists of a high-voltage power supply (SpellmanCZE1000R), a fused silica capillary (75 μm i.d., 360 μm o.d., PolymicroTechnologies, Phoenix, Ariz.) and a linear 200 UV/VIS monitor suppliedwith a deuterium lamp and a capillary flow cell. The total length of thecapillary was 64.5 cm, with a 1 cm optical window at 40 cm from theanode. UV data was retrieved and stored using LabVIEW version 4.0.1software (National Instruments).

Example 8 Analysis of Hydrolysis Product by MALDI-TOF

The hydrolysis product from each conjugate was examined by MALDI-TOF todetermine if there was any dimerization caused by reactions betweenhydrolysis intermediates. Free lysozyme was used as control. Nodimerization was observed.

Experiment 9 Bioactivity Measurement of Reversible Lysozyme Conjugate

Bioactivity of free lysozyme, PEG conjugates of lysozyme and lysozymerecovered from hydrolysis of the conjugates were measured by an assayfrom the standard protocol of Sigma for hen's egg white (HEW) lysozymeEC.3.2.1.17. A solution containing the unmodified or PEG-modifiedlysozyme was diluted to 5.5 μg/ml in a 66 mM sdium phosphate buffer (pH6.24). A suspension of 1.5 mg Micrococcus lysodeikticus in 10 ml of 66mM phosphate buffer (pH 6.24) was allowed to equilibrate at roomtemperature until the absorbance at 450 nm was constant. Then 0.1 ml ofa lysozyme solution was placed in a 1 cm light path quatz cuvettecontaining 2.5 ml of the substrate suspension. The decrease in theabsorbance at 450 nm was recorded and the activity was determined fromthe maximum linear rate. Eighty-two percent of lysozyme bioactivity wasrecovered from the m-PEG-lysozyme conjugate, while the ,PEG lysozyme hadundetectable bioactivity prior to hydrolysis.

Example 10 Preparation of Hydrogels from Di-Functional PEG 3400benzamide-m-succimidyl carbonate

In a test tube, 55 mg of di-functional PEG 3400 benzamide-m-succimidylcarbonate was dissolved in 0.36 ml of cold de-ionized water (4° C.).Then 0.36 ml of 8-arm-PEG amine 10,000 (Shearwater Polymers, Inc, AL,USA) solution (110 mg/ml, in pH 7 phosphate buffer) was added. Afterrapid mixing, the solution was allowed to stand at room temperature. Aclear gel formed in a few minutes.

Example 11 Degradation of the Hydrogels Prepared from Di-Functional PEGbenzamide-m-succimidyl carbonate

An approximately 0.2 cm³ piece of gel prepared from Example 8 was putinto about 1 ml of PBS buffer, while the other was put into the sameamount of human serum. Both samples were incubated at 37° C. Geldegradation was monitored visually to evaluate the degradation lifetimes. The gel was observed to degrade to yield a clear solution inapproximately 4 hours.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it will be obvious that certain changes and modificationsmay be practiced within the scope of the appended claims.

1. A compound having the formula:

wherein: POLY is a water soluble, non-peptidic polymer; L is a linkinggroup; and Ar is an aromatic group.
 2. The compound of claim 1, whereinsaid POLY is a poly(ethylene glycol) having a molecular weight of fromabout 200 to about 100,000 Daltons.
 3. The compound of claim 1, whereinsaid POLY further comprises a capping group selected from the groupconsisting of OH, alkoxy, and

wherein L′ is a hydrolytically stable linkage, and Ar′ is an aromaticgroup.
 4. The compound of claim 1, wherein L is a hydrolytically stablelinking group.
 5. The compound of claim 1, wherein L comprises a linkinggroup selected from the group consisting of ethers, amines, imides,esters, amides, carbamides, imides and thioethers.
 6. The compound ofclaim 1, wherein L is —O— or —HN—CO—.
 7. The compound of claim 1,wherein Ar is selected from phenyl, substituted phenyl, biphenyl,substituted biphenyl, polycyclic aryls, substituted polycyclic aryls,heterocyclic aryls, and substituted heterocylic aryls.
 8. A compoundhaving the formula:

wherein: L is —O— or —NHCO—; POLY is poly(ethylene glycol) having acapping group selected from the group consisting of —OH, alkoxy, and

wherein L is as described.
 9. A compound having the formula:

wherein mPEG is methoxy end-capped poly(ethylene glycol) and L is —O— or—NHCO—.
 10. The compound of claim 9, having the formula:

wherein mPEG is methoxy end-capped poly(ethylene glycol).
 11. A compoundhaving the formula:

wherein mPEG is methoxy end-capped poly(ethylene glycol) and L is —O— or—NHCO—.
 12. The compound of claim 11, having the formula:

wherein mPEG is methoxy end-capped poly(ethylene glycol).